This application is the national phase of PCT/EP00/05739, filed Jun. 21, 2000.
The present invention relates to a process for preparing 1,6-hexanediol which has a purity of at least 99% and is, in particular, essentially free of 1,4-cyclohexanediols from a carboxylic acid mixture obtained in the oxidation of cyclohexane to cyclohexanone/cyclohexanol using oxygen or oxygen-containing gases after water extraction of the reaction mixture followed by extraction with aqueous sodium hydroxide solution, which comprises neutralization of the extract, fractionation of the organic phase into a low boiler fraction and a high boiler fraction, esterification of the latter, fractionation of the esterification mixture to give an ester fraction which is free of 1,4-cyclohexanediols, hydrogenation of this ester fraction and purification of the 1,6-hexanediol by distillation. EP-A 883 590 discloses a process for preparing 1,6-hexanediol from a carboxylic acid mixture comprising adipic acid, 6-hydroxycaproic acid and small amounts of 1,4-cyclohexanediols which is obtained as by-product in the oxidation of cyclohexane to cyclohexanone/cyclohexanol using oxygen or oxygen-containing gases after water extraction of the reaction mixture, by esterification of the acids and hydrogenation, in which
a) the monocarboxylic and dicarboxylic acids present in the aqueous dicarboxylic acid mixture are reacted with a low molecular weight alcohol to give the corresponding carboxylic esters,
b) the esterification mixture obtained is freed of excess alcohol and low boilers in a first distillation step,
c) the bottom product is, in a second distillation step, fractionated to give an ester fraction essentially free of 1,4-cyclohexanediols and a fraction comprising at least the major part of the 1,4-cyclohexanediols,
d) the ester fraction which is essentially free of 1,4-cyclohexanediols is catalytically hydrogenated and
e) 1,6-hexanediol is isolated from the hydrogenation product in a manner known per se in a final distillation step.
The aqueous solutions of carboxylic acids employed as starting material in this process are formed as by-products in the oxidation of cyclohexane to cyclohexanol and cyclohexanone (cf. Ullmann""s Encyclopedia of Industrial Chemistry, 5. Ed, 1987, Vol. A8, pp. 2/9); they will hereinafter be referred to as dicarboxylic acid solution (DCS). They generally comprise (calculated on an anhydrous basis in % by weight) from 10 to 40% of adipic acid, from 10 to 40% of 6-hydroxycaproic acid, from 1 to 10% of glutaric acid, from 1 to 10% of 5-hydroxyvaleric acid, from 1 to 5% of 1,2-cyclohexanediols (cis and trans), from 1 to 5% of 1,4-cyclohexanediol (cis and trans), from 2 to 10% of formic acid and also many further monocarboxylic and dicarboxylic acids, esters, oxo and oxa compounds whose individual contents generally do not exceed 5%. Examples which may be mentioned are acetic acid, propionic acid, butyric acid, valeric acid, caproic acid, oxalic acid, malonic acid, succinic acid, 4-hydroxybutyric acid and gamma-butyrolactone.
The water scrub is generally followed by a further scrub using aqueous sodium hydroxide solution which gives a waste stream consisting of an alkaline solution of salts of a large number of different carboxylic acids, known as xe2x80x9ccaustic waterxe2x80x9d.
The formation of this stream is described in Ullmann""s Encyclopedia of Industrial Chemistry, 5th Ed. 1987, Vol. A, p. 219.
In contrast to the DCS which predominantly comprises readily water-soluble acids such as dicarboxylic acids, hydroxycarboxylic acids and diols, this caustic water stream comprises mostly less polar carboxylic acids, e.g. linear unsubstituted monocarboxylic acids and, in much smaller amounts, C6 building blocks such as adipic acid and hydroxycaproic acid. Since there has up to now been no known economic use of the caustic water, it has to be disposed of. This is achieved in Europe by incineration of the salts to form xe2x80x9cheavy sodaxe2x80x9d (Na2CO3). In the USA, disposal is also carried out by means of xe2x80x9cdeep wellingxe2x80x9d, i.e. injection into deep geological layers. The latter is undesirable from an ecological point of view and in view of the rule of sustainability.
It is an object of the present invention to find a better use for the caustic water so as to obtain a useful product.
We have found that this object is achieved by using the caustic water for preparing 1,6-hexanediol by the process of EP-A 883 590 when the carboxylic acids present in the alkaline extract are liberated by addition of a mineral acid, preferably sulfuric acid, the organic phase is separated off and separated into a low boiler fraction consisting essentially of the monocarboxylic acids and a high boiler fraction comprising about 30% of C6-dicarboxylic acids and 6-hydroxycaproic acid, and this fraction is processed further by the process described.